Intracardiac bio-impedance is a parameter correlated with cardiac flow, and thus with the ejection fraction, which are parameters that can be useful in controlling certain stimulation parameters such as heart rate and/or atrio-ventricular delay in a direction making it possible to maximize flow, or in controlling inter-ventricular delay in the case of a biventricular stimulation.
EP-A-1116497 and its counterpart U.S. Pat. No. 6,604,002 (which U.S. patent is incorporated herein by reference), both assigned herewith to ELA Médical, Montrouge, France, describe a dynamic measurement of intracardiac trans-valvular bio-impedance (i.e., between an atrium and a ventricle located on the same side of the heart). EP-A-1138346 and its counterpart U.S. Pat. No. 6,725,091 (which U.S. patent is incorporated herein by reference), both assigned herewith to ELA Médical, Montrouge, France, describe measurement of a trans-septum bio-impedance (i.e., between a site located on one side of the heart and a site located on other side of the heart), such configuration capable of being an oblique trans-septum configuration (i.e., between a ventricle on one side of the heart and an atrium located on the opposite side) or an interventricular trans-septum configuration (i.e., between the two ventricles).
The intracardiac bio-impedance is measured by injection of a current and collection of a voltage at respective poles in a tripolar or quadripolar configuration of electrodes placed inside the myocardium (atrial electrode, ventricular proximal electrode, ventricular distal electrode, etc). The bio-impedance, no matter how it is measured, is a dynamic parameter (i.e., it varies continuously during the same cardiac cycle) giving an indication of the instantaneous cardiac flow. More precisely, variations of intracardiac bio-impedance depend mainly on variations of the volume of the cardiac cavities, a low impedance corresponding to a high volume, and a high impedance corresponding to a low volume. The impedance thus varies between a minimum, reached at the end of the diastolic phase, and a maximum, reached at the end of the systolic phase. The difference between the systolic impedance and the diastolic impedance gives a value correlated to the ejection volume, a value from which one can evaluate the cardiac flow, which is the product of the ejection volume multiplied by the heart rate. The indications provided by these measurements are relative indications of the systolic and diastolic volumes, i.e., one determines the difference between these volumes, which, with the knowledge of the rate, is sufficient if one only wishes to evaluate the cardiac flow. On the other hand, these devices do not give access to an absolute measurement of intracardiac volume.
The starting point of the present invention is the observation that, in certain circumstances, it may be useful for diagnostic purposes to have a signal representing an absolute measurement of intracardiac volume. Until now, the absolute value of intracardiac volume has been estimated starting from echographic signals, which make it possible to evaluate the diastolic volume and the systolic volume, i.e., the two extreme values of intracardiac volume, and gives an indication of the way in which volume varies between these two extremes during the same cycle. However, this technique is applicable only when the patient undergoes an examination, which does not allow a permanent follow-up, over the long term. Further, the values obtained are computed values, which are posted or printed by the echographic device but do not constitute signals that could, for example, allow controlling of a function of a pacemaker or a diagnosis of certain pathologies by analysis of evolution of this signal.